The invention relates to a process for the production of water-soluble, powdered, cationic polyelectrolytes based on non-ionogenic and cationic monomers.
Copolymers of acrylamide and cationic monomers are used in waste water treatment and paper manufacturing, for example. It is their purpose to coagulate colloidal particles in aqueous suspensions to form mechanically stable flakes which settle readily or can easily be filtrated.
It is desirable for economic reasons to achieve high settling rates and high filtration rates of the coagulated particles with a minimum of polyelectrolyte input. High effectiveness in the clarification of aqueous suspensions is presented by high molecular weight cationic polyelectrolytes having good solubility.
High molecular weight cationic water-soluble copolymers are obtained when the polymerization of the aqueous monomer solution is performed at a polymerization temperature as low as possible and with small amounts of initiator.
As the viscosity of the monomer solution increases after initiating the polymerization, and a solid gel is present after a short period of polymerization, controlled heat removal is not possible, i.e., the exothermic reaction proceeds in a virtually adiabatic fashion. In order to keep the maximum polymerization temperature as low as possible, the temperature of the monomer solution at the time of initiation must be selected low at a given monomer concentration. In summary, this means that in order to produce high molecular weight cationic polymers by polymerization in aqueous solution, not only the initiation temperature and the amount of initiator, but also the monomer concentration must be kept low, the amount of initiator ranging approximately between 0.02 and 5 wt.-% and the monomer concentration approximately between 25 and 45 wt.-%. Such preconditions result in extensive problems when producing water-soluble, high molecular weight copolymers from non-ionogenic and cationic monomers because:
1) initiator systems consisting of an oxidizing agent and a reducing agent (redox system) which are sufficiently active to initiate polymerization at temperatures below 20xc2x0 C., preferably below 0xc2x0 C., are available in only a limited range;
2) redox systems initiating polymerization at low input and low initiation temperature are not capable of directing the polymerization to high conversion;
3) low amounts of initiator require exceedingly long periods of polymerization which do not permit a continuous design of the polymerization process;
4) low amounts of initiator result in poorly reproducible initiation and progress of the polymerization because even minor variations in the monomer quality or oxygen content of the monomer solution will give rise to significant interference with the process;
5) polymerization at low monomer concentrations not only deteriorates the space-time yield but also impedes breaking up of the hydrous, gelled polymer into separate particles, hampering the subsequent drying.
To overcome the above-demonstrated problems in the production of high molecular weight, water-soluble polymers, the EP 0,296,331 suggests initiation of the polymerization at temperatures below 0xc2x0 C. in the presence of dispersed monomer crystals. The polymerization is initiated using the well-known redox system consisting of ammonium persulfate and ammonium iron(II) sulfate which reacts sensitively to traces of oxygen. This redox system does not provide sufficient conversion, however, and for that reason 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride (ABAH) is also added, which provides free radicals by thermal decomposition. The thermal decomposition of this azo initiator becomes apparent by an accelerated polymerization from about 45xc2x0 C. on, and as a consequence, the molecular weight of the polymers formed decreases with increasing polymerization temperature. The residual monomer content of acrylamide is 970 ppm, and such high values in the polymer cannot be accepted for toxicological reasons.
In order to produce high molecular weight polyacrylamides by polymerizing acrylamide or acrylamide including other comonomers in aqueous solution, the U.S. Pat. No. 4,455,411 claims an initiator system consisting of a peroxodisulfate and the sodium formaldehyde sulfoxylate (=sodium hydroxymethanesulfinic acid=Rongalit C(copyright)) reducing agent, as well as the 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride (ABAH) already mentioned above. Also, the initiator system appears to be insensitive to varying purity of the monomer used. Example 8 describes the production of a cationic polyelectrolyte. The copolymerization of acrylamide with quaternized dimethylaminoethyl methacrylate is initiated with the redox system at 20xc2x0 C. and carried on to high conversion by using 403 ppm. of ABAH.
As has been illustrated, the production of high molecular weight, well-soluble polyelectrolytes necessarily requires a low initiation temperature, i.e., a temperature of the monomer solution of below 20xc2x0 C., preferably below 10xc2x0 C.
In contrast to redox polymerization, the photopolymerization of a monomer solution in the presence of an initiator providing free radicals by irradiation with UV light is largely independent of the initiation temperature (Chemical Reviews, Vol. 68, No. 2, Mar. 25, 1968). The disadvantage of photopolymerization results from the Lambert-Beer Law which states that the light intensity in the irradiated monomer solution decreases exponentially with increasing layer thickness. For this reason, photochemical reactions should be stirred vigorously in order to replace reacted product in the reaction zone by new substrate.
In continuous processes for producing high molecular weight, water-soluble polymers, such as described in EP 0,296,331 A2, Example 4, intimate mixing of the polymerizing solution is not possible because immediately after starting the polymerization initiated by a redox initiator system of iron ammonium sulfate/ammonium persulfate with addition of an azo initiator, it would solidify to give a solid gel which can no longer be stirred.
The DOS 27 16 606 suggests performing the continuous production of water-soluble acrylic polymers by photopolymerization, preferably at a layer thickness of the monomer solution of from 3 to 8 mm on a mobile support. In order to decrease the residual monomer content of the polymers obtained compared to those polymers obtained according to the process of DAS 2,050,988, it is necessary according to claim 1d) to irradiate the polymer layer after removal from the support for preferably another 40 to 90 minutes. According to Example 4, the residual content of acrylamide in the polymer is 600 ppm despite the use of 240 ppm of benzoin propyl ether and 75 minutes irradiation time. Such long polymerization periods with thin layer thickness result in poor space-time yields. The tackiness of the polymers obtained as determined by the applicant is a consequence of the thin polymerizing layer, i.e., the unfavorable ratio of volume to surface results in an increase of low molecular weight tacky components forming at the phase boundary, particularly towards the gaseous phase.
To produce water-soluble, low molecular weight polymers, the DOS 22 48 715 suggests performing the photopolymerization in a thin layer using benzoin propyl ether and an accelerator. The accelerator consists of azobisisobutyronitrile or the persulfate/bisulfite redox system.
A process for the continuous production of polymers and copolymers of water-soluble monomers is known from EP 0,228,638 A1, wherein polymerization of the aqueous monomer solution takes place between xe2x88x9210xc2x0 C. and 120xc2x0 C. on a mobile conveyor belt, initiated by chemical initiation and/or high-energy radiation/light. Polymers having a low content of residual monomer and gel have not been described.
Cationic copolymers of acrylamide and dimethylaminopropylacrylamide as flocculant can be inferred from EP 0,228,637 B1. The initiation of polymerization can be triggered using redox systems, thermally decomposing initiators, or free radicals formed by photochemical means.
Crosslinked polymers absorbing aqueous liquids are known from DE 40 15 085 C2, which are characterized by a high absorptive capacity under pressure and a low residual monomer content. According to this invention, polymerization is initiated using a redox initiator system of formamidinosulfinic acid and organic peroxides; in addition, the polymerization may also be initiated using UV light. Azo compounds may also be added as additional components of the polymerization initiators. Water-soluble polymers have not been described in this patent.
Referring to the state of the art, there is no process known for producing cationic water-soluble polyelectrolytes, which would initiate polymerization at low temperatures in a short, technically acceptable period of time, permit formation of polymers having high molecular weights of more than 1 million andxe2x80x94avoiding large amounts of expensive azo initiatorsxe2x80x94exhibit low ratios of residual monomers and gel.
It is the object of the present invention to provide a process for the production of high molecular weight, water-soluble, cationic polymers, permitting polymerization of monomer solutions with a layer thickness of more than 1 cm at initiation temperatures of below 20xc2x0 C., preferably below 10xc2x0 C. This new process should be largely insensitive to residual oxygen in the monomer solution and varying monomer quality.
The new adiabatic polymerization process should be controllable both in the lower and upper temperature ranges. According to the new process, the content of toxic residual monomers in the polymer should be below 500 ppm, preferably below 100 ppm, without requiring addition of large amounts of expensive azo initiator.
Said object was accomplished by irradiating a monomer solution with light at a temperature of below 20xc2x0 C. in the presence of A) a photoinitiator and B) a redox system, characterized in that the redox system is capable of initiating the polymerization only at temperatures higher than or equal to 20xc2x0 C.
In redox system B), hydroperoxides such as tert-butyl hydroperoxide and cumene hydroperoxide are used as oxidizing agents. Conventionally, from 1 to 1000 ppm, preferably from 10 ppm to 500 ppm, and more preferably from 10 ppm to 200 ppm of organic hydroperoxide is used, based on monomer solution. Suitable as reducing agents are sulfinic acids or salts of sulfinic acids such as sodium hydroxymethanesulfinate, p-toluenesulfinic acid and formamidinosulfinic acid. Here, amounts of from 0.1 to 1000 ppm, preferably from 1 to 500 ppm, and more preferably from 10 to 250 ppm are conventionally used, based on monomer solution.
For example, benzoin derivatives such as benzoin isopropyl ether or benzil dimethyl ketal, azo initiators such as 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride or 2,2xe2x80x2-azobisisobutyronitrile are used as photoinitiators forming free radicals upon exposure to light, which have a polymerization-initiating effect. The amount of photoinitiators is from 5 to 500 ppm, preferably from 5 to 100 ppm.
Despite of rapid gelling of the polymerizing monomer solution, the adiabatic polymerization process according to the invention is largely controllable. Within a temperature range of below 30xc2x0 C., the progress of polymerization, i.e., the amount of free radicals formed, can be determined by way of type and concentration of the photoinitiator as well as the irradiation intensity. Thus, in contrast to other initiator systems, the surprisingly high insensitivity of the process according to the invention to residual amounts of oxygen in the monomer solution enables initiation of the polymerization without delay and therefore, a higher production rate as well.
As a result of good controllability of the polymerization process, it is also possible to maintain the concentration of the monomer solution in the upper range, with no unintended, difficult to control temperature peaks occurring.
Another advantage of the process according to the invention is that it is possible to disperse the initiators thoroughly in the monomer solution during the continuous production process, e.g., in a static mixer, with no premature initiation of polymerization and thus, blocking of the metering systems occurring prior to applying the monomer solution onto the polymerization belt.
Initiation of polymerization is effected by irradiating the monomer solution cooled down to below 20xc2x0 C., preferably below 10xc2x0 C., and more preferably below 0xc2x0 C. It is preferred to irradiate using UV light having a wavelength of from 300 to 400 nm. Advantageously, UV lamps can be used where the radiation component has a maximum at a wavelength of 360 nm (e.g., TL09, Philips Company).
The layer thickness of the irradiated monomer solution is at least 1 cm, preferably more than 2 cm, with more than 4 cm being particularly preferred.
As a result of the exothermic reaction initiated by irradiating the monomer solution, the temperature in the polymerizing solution rises, reaching the initiation temperature of the redox system above 20xc2x0 C. The amount of the redox components and the ratio of oxidizing agents to reducing agents will determine the progress of polymerization at elevated temperatures, so that the molecular weight of the polymers is not decreased by free radicals present in excess and yet, high conversion is achieved at the same time.
Surprisingly, high molecular weights of more than 1 million and at the same time, residual contents of toxic monomer of less than 200 ppm, preferably less than 100 ppm, and more preferably less than 50 ppm are achieved in the claimed photopolymerization process in the presence of a redox system.
According to the new process, water-soluble, high molecular weight, cationic copolymers can be produced in particularly high quality by polymerizing cationic monomers such as N,N-dialkylaminoalkyl (meth)acrylates, N,N-dialkylaminoalkyl(meth)acrylamides as salts or in quaternized form with non-ionogenic monomers such as acrylamide, methacrylamide and N-alkyl(meth)acrylamides.
The ratio of cationic to non-ionogenic monomers is from 98:2 to 2:98 wt.-%.
Uncontrolled effects of heavy metal salts such as Cu, Fe or Mn ions on the redox system can largely be eliminated by adding chelating substances to the monomer solution. Examples of such chelating substances are diethylenetriamine-pentaacetic acid (Versenex(copyright), DTPA) and ethylenediaminetetra-acetic acid (EDTA).
Preferably, the polymerization process is performed continuously in boxes situated on a moving support or on an elastic conveyor belt pressed by lateral rolls into concave shape across the conveying direction, so that the monomer solution may form a layer more than 1 cm in height. The belt unit is situated in a tunnel-like housing from which the vapors formed are sucked off through a waste gas washer.
In order to perform the photopolymerization, individually switchable UV fluorescent lamps adjustable in height are installed at the housing top.
One drawback of this continuous plant is that the monomer solution in the tray- or box-shaped open-top polymerization tank is in direct contact with the gaseous phase in the tunnel housing, which cannot be maintained free of oxygen. Although the oxygen content of the monomer solution before entering the polymerization plant is reduced to below 1 ppm by purging with nitrogen, it will increase again by exchange with the gaseous phase in the tunnel in the top layer of the monomer solution, thereby inhibiting polymerization and reducing conversion, respectively. This drawback is overcome by the process claimed according to the invention, so that the polymerization can be effected even at an elevated oxygen content in the monomer solution of 1 ppm, for example, without a time-delaying inhibition phase.
Following polymerization, the hot gel block is pre-crushed in a kneader and optionally treated with oxidation stabilizers, pH stabilizers or conditioners as described in DE 41 23 889 A1 and DE 41 27 814 A1 , for example. In particular, treatment of the polymer gel according to the procedure of DE 37 24 709 A1 is preferred in order to reduce the ratio of insolubles (gel fraction) and further decrease the residual monomer content. Following granulation to a grain size of from 1 to 5 mm in an extruder, the gel is dried in a stream of hot air and subsequently milled.
The cationic polymers produced according to the process of the invention are particularly advantageous in the purification of municipal and industrial waste waters, as well as in flocculation procedures in paper manufacturing, because they have excellent flocculating activity and low contents of toxic residual monomer.